1. Field of the Invention
The present invention relates to a reflection reduction projection optical system. More particularly, the present invention relates to a reflection reduction projection optical system for use in X-ray lithography.
2. Description of the Related Art
Reflection reduction projection optical systems for use in X-ray lithography are known. For example, Japanese Laid-Open Patent Publication JP-A-Hei 5-36588 discloses a reflection reduction projection optical system using four (4) aspheric surface reflecting mirrors. However, the optical system disclosed in JP-A-Hei 5-36586 suffers from the problem that the numerical aperture (xe2x80x9cNAxe2x80x9d) can not be made large.
It is an object of the present invention to solve the problems of the known reflection reduction projection optical system, and to provide a reflection reduction projection optical system having a large numerical aperture for use in X-ray lithography.
Objects and advantages of the present invention are achieved in accordance with embodiments of the present invention with a reflection reduction projection optical system comprising at least five (5) aspheric surface reflecting mirrors and a reflecting mirror having a predetermined configuration, wherein the at least five aspheric surface reflecting mirrors and the reflecting mirror having a predetermined configuration perform reduction imaging on an imaging surface by imaging a light beam from an object only one time.
In accordance with embodiments of the present invention, the reflection reduction projection optical system preferably comprises, in light path sequence from the object side, a first convex mirror, a first concave mirror, the mirror having a predetermined configuration, a second concave mirror, a second convex mirror, and a third concave mirror.
In accordance with embodiments of the present invention, the reflection reduction projection optical system preferably satisfies the following condition (1):
xe2x88x920.005 less than p1+p2+p3+p4+p5+p6 less than 0.005, 
wherein p1 is the inverse of the focal distance of the first convex mirror, p2 is the focal distance of the first concave mirror, p3 is the inverse of the focal distance of the reflecting mirror having the predetermined configuration, p4 is the focal distance of the second concave mirror, p5 is the inverse of the focal distance of the second convex mirror, and p6 is the focal distance of the third concave mirror.
Objects and advantages of the present invention are achieved in accordance with embodiments of the present invention with a reflection reduction projection optical system comprising a plurality of reflecting mirrors which image a light beam from an object on an imaging plane, the plurality of reflecting mirrors including three reflecting mirrors arranged in an upstream direction from the imaging plane in a light path, wherein the three reflecting mirrors satisfy the following condition (2):
0xc2x0xe2x89xa6Umaxxe2x88x92Uminxe2x89xa68.8xc2x0, 
where Umax is an angle of incidence of a light ray which is incident on the reflecting mirror at the largest angle of incidence, and Umin is an angle of incidence of a light ray which is incident on the reflecting mirror at the smallest angle of incidence.
Objects and advantages of the present invention are achieved in accordance with embodiments of the present invention with a reflection projection optical system comprising only reflecting mirrors which respectively satisfy the condition (2):
0xc2x0xe2x89xa6Umaxxe2x88x92Uminxe2x89xa68.8xc2x0, 
wherein Umax is the angle of incidence of a light ray which is incident on the reflecting mirror at the largest angle of incidence, and Umin is an angle of incidence of a light ray which is incident on the reflecting mirror at the smallest angle of incidence.
In accordance with embodiments of the present invention, the reflecting projection optical system preferably includes at least five (5) aspheric surface reflecting mirrors and a reflecting mirror having a predetermined configuration.
In accordance with embodiments of the present invention, a reflecting reduction projection optical system is provided which has a large NA for use in X-ray lithography.